Adhesive tape and process for its production

ABSTRACT

An adhesive tape comprises a base sheet of a polypropylene resin, an adhesive layer formed on one side of the base sheet and a roughened surface layer formed on the other side of the base sheet and composed of a polypropylene resin containing fine inorganic filler particles, in which the roughened surface layer has a thickness of not more than the maximum particle diameter of said fine inorganic filler particles and is substantially free from voids or cracks, and the total percent light transmittance of the tape is at least 90%. Such an adhesive tape is produced by a process which comprises forming an adhesive layer on one side of a base sheet of a polypropylene resin and a surface layer of a polypropylene resin having a melting point lower than that of the polypropylene resin of the base sheet and containing fine inorganic filler particles, on the other side of the base sheet to obtain an unstretched laminated sheet, and then stretching the laminated sheet at a temperature of at least the melting point of the polypropylene resin of the surface layer to bring the thickness of the surface layer to be not more than the maximum particle diameter of said fine inorganic filler particles.

This is a division of application Ser. No. 397,237, filed July 12, 1982,now U.S. Pat. No. 4,447,485.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an adhesive tape having excellentwritability and photostatic property and a process for its production.

2. Description of the Prior Art

An adhesive tape of this type is generally called a mending adhesivetape and useful for mending e.g. drawings. The mending adhesive tape isrequired to be writable by e.g. a pencil (hereinafter referred to as"writability") and tearable by fingers in the transverse direction ofthe tape (hereinafter referred to as "finger-tearability") and to haveexcellent light transmittance so as to permit photostatic reproductiontherethrough (hereinafter referred to as "photostatic property").

Heretofore, in order to improve primarily the writability andfinger-tearability among the required properties for a mending adhesivetape, it has been proposed to incorporate fine inorganic fillerparticles in the base sheet of an adhesive tape, and such a tape isactually commercially available. However, in such a tape, lightscattering is likely to be caused by the inorganic filler particles perse or by voids or cracks formed at the interface between the inorganicfiller particles and the base sheet resin, and the total percent lighttransmittance of the tape tends to be thereby reduced. Thus, such a tapeis inferior in the photostatic property and is not suitable as a mendingadhesive tape.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above mentioneddrawbacks of the conventional adhesive tape and to provide an adhesivetape having excellent writability and photostatic property required fora mending adhesive tape.

Another object of the present invention is to provide a process forproducing such a superior adhesive tape.

Namely, the present invention provides an adhesive tape comprising abase sheet of a polypropylene resin, an adhesive layer formed on oneside of the base sheet and a roughened surface layer formed on the otherside of the base sheet and composed of a polypropylene resin containingfine inorganic filler particles, in which the roughened surface layerhas a thickness of not more than the maximum particle diameter of saidfine inorganic filler particles and is substantially free from voids orcracks, and the total percent light transmittance of the tape is atleast 90%.

Such an adhesive tape is produced by a process which comprises formingan adhesive layer on one side of a base sheet of a polypropylene resinand a surface layer of a polypropylene resin having a melting pointlower than that of the polypropylene resin of the base sheet andcontaining fine inorganic filler particles, on the other side of thebase sheet to obtain an unstretched laminated sheet, and then stretchingthe laminated sheet at a temperature of at least the melting point ofthe polypropylene resin of the surface layer to bring the thickness ofthe surface layer to be not more than the maximum particle diameter ofsaid fine inorganic filler particles.

Other objects or features of the present invention will be apparent fromthe following detailed description of the preferred embodiments of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the composition of the roughened surfacelayer in which the effective ranges of the components constituting theroughened surface layer are indicated by oblique lines.

FIGS. 2 to 5 are diagrammatic cross sectional views of adhesive tapesaccording to the present invention, wherein the adhesive layer isomitted.

DETAILED DESCRIPTION OF THE INVENTION

The polypropylene resin to be used for the base sheet may be anisotactic polypropylene commonly used in films and may also be acopolymer of a propylene with other α-olefin. In order to obtain goodfinger-tearability,it is preferred to use a mixture of a polypropylenewith a methylpentene polymer. The mixture is preferably composed of from95 to 50% by weight of a polypropylene and from 5 to 50% by weight of amethylpentene polymer. If the proportions are outside the above ranges,the finger-tearability tends to be inferior. The methylpentene polymershould preferably have a Vicat softening point (as measured by JISK-7206) lower than the melting point of the polypropylene to obtain goodtransparency of the base sheet. The methylpentene polymer may also be acopolymer with a small amount of other α-olefin.

A roughened surface layer composed of a polypropylene resin containingfine inorganic filler particles is formed on one side of the base sheetto provide excellent writability. The polypropylene resin used for theroughened surface layer has a melting point lower than the polypropyleneresin used for the base sheet. As such a resin, there may be mentioned acopolymer of propylene with other α-olefin such as ethylene. The fineinorganic filler particles may be those commonly used as inorganicfillers for plastics, such as calcium carbonate or clay. It is preferredthat the fine inorganic filler particles have a maximum particlediameter of about 10μ, and they are capable of forming a roughenedsurface having a maximum roughness of at most 10μ and an averageroughness of 10 points of from 2.0 to 6.0μ as measured by JIS B-0601.

It is preferred that the roughened surface layer is composed of from 90to 50% by weight of the polypropylene resin and from 10 to 50% by weightof the fine inorganic filler particles.

In order to obtain good writability, it is important that the roughenedsurface layer has a thickness of not more than the maximum particlediameter of the fine inorganic filler particles. In order to ensure thedesired transparency and to obtain a good photostatic property, it isalso important that the roughened surface does not contain internalvoids or cracks. Such a roughened surface layer can be advantageouslyformed by the process which will be described hereinafter.

A paper surface to be mended by an adhesive tape is usually deglazed,and accordingly, in order to obtain a satisfactory photostat copy onwhich the configuration of the afixed tape is scarcely reproduced orcopied, it is desired that the surface of the adhesive tape is likewisedeglazed as far as possible (hereinafter referred to as"non-glossiness"). Therefore, the "good non-glossiness" means that therelative specular glossiness is small and the specular reflection oflight on the surface is minimum. In order to impart the non-glossinessto the adhesive tape, a specific thermoplastic resin may preferably beincorporated in the roughened surface layer.

According to a preferred embodiment as illustrated in FIG. 1, theroughened surface layer is composed of from 25 to 85% by weight of apolypropylene resin (X), from 10 to 50% by weight of fine inorganicfiller particles (Y) and from 5 to 45% by weight of one or morethermoplastic resins (Z) selected from the group consisting of amethylpentene polymer, a polystyrene, a polyalkyl methacrylate and apolyester, provided that the total of the components (X), (Y) and (Z) isalways 100% and the amount of the component (Z) is not more than theamount of the component (X), whereby the relative specular glossiness ofthe roughened surface layer is not more than 30%.

If the component (X) is less than 25% by weight, voids or cracks arelikely to form which impair the photostitic property. On the other hand,if the component (X) exceeds 85% by weight, the writability will beinferior.

If the component (Y) (i.e. fine inorganic filler particles) is less than10% by weight, the writability will be inferior, and if it exceeds 50%by weight, the photostatic property will be inferior.

Referring to the specific thermoplastic resin (Z), the methylpentenepolymer may be the same resin as used in the base sheet. As thepolyester, there may be mentioned polyethylene terephthalate orpolybutylene terephthalate. The polystyrene may be a so-called modifiedpolystyrene wherein a small amount of a carboxylic acid anhydride or itsderivative is incorporated. The component (Z) is incorporated within arange of from 5 to 45% by weight. If it is less than 5% by weight, thenon-glossiness will be inadequate, and if it is more than 45% by weight,the photographic property will be inferior. Further, the amount of thecomponent (Z) must be not more than the amount of the component (X). Ifthe component (Z) exceeds the amount of the component (X), voids orcracks are likely to form.

The relationship among the amounts of the respective components (X), (Y)and (Z) is illustrated in FIG. 1, wherein the effective ranges of thecomponents are shown by oblique lines.

It is desirable that an adhesive tape can easily be cut straight in thetransverse direction by fingers or by a dispenser cutter (i.e. a tapecutter). It is frequently experienced that an adhesive tape is tornobliquely at an intermediate portion of its width depending upon theforce applied to cut or tear it by a dispenser cutter or fingers.

It has now been found possible to obtain the straight transversetearability or severability and to avoid the undesirable obliquetearing, by forming a crystalline polypropylene layer on one side oreach side of the polypropylene/a methylpentene polymer mixture layer ofthe base sheet.

Namely, according to the preferred embodiments of the inventionillustrated in FIGS. 2 to 5, the base sheet comprises a main layer (A)composed of from 95 to 50% by weight of a polypropylene and from 5 to50% by weight of a methylpentene polymer and a subsidiary layer (B)formed on one side or each side of the main layer (A) and composed of acrystalline polypropylene having a melting point of at least the Vicatsoftening point of said methylpentene polymer, and a bonding layer (D)of a carboxylic acid-modified polypropylene is interposed between thebase sheet and the adhesive layer. In FIGS. 2 to 5, symbol (C)designates the roughened surface layer.

The main layer (A) of the base sheet is composed of a resin mixture ofthe polypropylene and the methylpentene polymer i.e.poly-4-methyl-pentene-1, in the above specified ratio. The polypropyleneto be used for this main layer (A) (hereinafter referred to as "A-layerPP") is a propylene homopolymer or a propylene copolymer containing asmall amount of a copolymerized component, and it may be the onecommonly used as a starting material for films. The methylpentenepolymer, which may be a copolymer with a small amount of other α-olefin,is preferably the one having a Vicat softening point (as measured by JISK-7206) lower than the melting point of the A-layer PP to obtain goodtransparency of the main layer (A). As mentioned above, it is preferredto mix from 95 to 50% by weight of the A-layer PP and from 5 to 50% byweight of the methylpentene polymer. If the mixing ratio is outsidethese ranges, the tape thereby obtained tends to be hardly torn byfingers.

In order to improve the severability by a dispenser cutter, acrystalline polypropylene layer (B) is formed on one side or each sideof the main layer (A) as shown in FIGS. 2 to 4 or as an intermediatelayer between two main layers (A) as shown in FIG. 5. As the crystallinepolypropylene (hereinafter referred to as "B-layer PP") constitutingthis layer (B), there may be used a propylene homopolymer, a propylenecopolymer containing a small amount (i.e. not more than about 3% byweight) of a copolymerized component (e.g. ethylene), or a carboxylicacid-modified polypropylene prepared by grafting thereto a carboxylicacid. The B-layer PP may be the same as the A-layer PP. In order toimprove the severability or tearability, it is necessary that theB-layer PP has a melting point of at least the Vicat softening point ofthe above mentioned methylpentene polymer. If the melting point is lessthan the Vicat softening point, no improvement of the severability by adispenser cutter is obtainable. Namely, as will be described in detailhereinafter, the stretching temperature for the preparation of theadhesive tape according to the present invention is determined by theVicat softening point of the methylpentene polymer in the main layer(A), and the stretching can most advantageously be carried out at atemperature of at least said softening point and at most the meltingpoint of the A-layer PP or the B-layer PP whichever is lower.

Accordingly, if the B-layer PP has a melting point of at least the abovementioned softening point, the B-layer PP is stretched without meltingor flowing at the time of the stretching operation, which is believed tocontribute the improvement of the severability by a dispenser cutter.

In general, the optimum stretching temperature of a polypropylene isusually within a range of about 150° to about 160° C. Therefore, it isadvantageous to use a methylpentene polymer having a Vicat softeningpoint within this range and to use a B-layer PP having a melting pointof at least the softening point, preferably at least 155° C. and morepreferably at least 160° C.

This layer (B) should not substantially contain other polymer or organicor inorganic particles which are not compatible with the B-layer PP.

The polypropylene to be used for the roughened surface layer (C)(hereinafter referred to as "C-layer PP") is preferably the one having amelting point lower than that of the above mentioned A-layer PP andB-layer PP. For instance, a propylene-ethylene copolymer having amelting point of not more than 155° C. is preferred.

In order to firmly bond the adhesive layer to the base sheet, a bondinglayer (D) of a carboxylic acid-modified polypropylene is interposedtherebetween in the preferred embodiments. The carboxylic acid-modifiedpropylene is prepared by grafting an unsaturated carboxylic acid such asmaleic acid or itaconic acid or an unsaturated carboxylic acid anhydridesuch as maleic acid anhydride to a polymer composed mainly of propylene.

If the carboxylic acid-modified polypropylene is used as the B-layer PPin the structure as shown in FIG. 3 or 4, this layer may serve also asthe layer (D).

With respect to the relative thicknesses of the layers (A) to (D), it isimportant that the total thickness of the layers (B) and (D), so long asthey have a melting point of at least the Vicat softening point of themethylpentene polymer in the layer (A), is within a range of from 5 to30%, preferably from 10 to 20%, of the total thickness of the layers (A)to (D). Namely, in a case where the carboxylic acid-modifiedpolypropylene of the bonding layer (D) has a melting point of at leastthe Vicat softening point of the methylpentene polymer in the main layer(A), the total thickness of the layers (B) and (D) is within the aboverange. Whereas, when a carboxylic acid-modified polypropylene havingmelting point lower than the Vicat softening point of the methylpentenepolymer is used as the layer (D), such a layer does not very muchcontribute to the improvement of the severability by a dispenser cutter,and in such a case, the layer(s) (B) alone should have a thicknesswithin the above range. If the above mentioned total thickness of thelayers having a melting point of at least the Vicat softening point ofthe methylpentene polymer is less than 5%, no adequate improvement ofthe severability by a dispenser cutter can be attained, i.e. it isimpossible to adequately eliminate the drawback that the tape is likelyto be torn obliquely when cut by a dispenser cutter. On the other hand,if the total thickness exceeds 30%, the finger-tearability tends to beinferior.

It is believed that the desirable straight transverse severability ortearability is imparted to the layer (B) or layers (B) and (D) bybiaxial stretching in both the longitudinal and transverse directions ata temperature of not higher than the melting point thereof, and theseverability in the transverse direction thereby imparted compensatesthe irregular severability or tearability of the layers (A) and (C).

Now, the process for producing the adhesive tape according to thepresent invention will be described.

Firstly, an adhesive layer is formed on one side of a base sheet of apolypropylene resin and a surface layer of a polypropylene resincontaining fine inorganic filler particles is formed on the other sideof the base sheet to obtain an unstretched laminated sheet. Thepolypropylene resin used for the surface layer has a melting point lowerthan the melting point of the polypropylene resin of the base sheet.Then, the laminated sheet is stretched at a temperature of at least themelting point of the polypropylene resin of the surface layer until thethickness of the surface layer is brought to be not more than themaximum particle diameter of the fine inorganic filler particles.

In the first step, the base layer and the surface layer mayadvantageously be laminated by co-extrusion or extrusion lamination andthen the adhesive material is applied to the base sheet on the sideopposite to the surface layer. The adhesive material is preferably anacrylic ester type adhesive material having good adhesioncharacteristics, and is preferably applied without using any solvent. Inthis case, it is preferred to preliminarily laminate a carboxylicacid-modified polyolefin layer on the base sheet in order to securelybond the adhesive material to the base sheet. It is advantageous thatthe carboxylic acid-modified polyolefin layer is co-extruded with thebase sheet and the surface layer to form a lamination.

Namely, in a preferred embodiment, a modified polyolefin containing from0.01 to 10% by weight of an unsaturated carboxylic acid or itsderivative is applied to the base sheet to form a bonding layer on theopposite side of the surface layer, and a mixture of an acrylic adhesivehaving a functional group and a cross-linking agent is applied onto thebonding layer without using any solvent.

The modified polyolefin may be the one obtained by addition-polymerizingor copolymerizing a polyolefin such as polyethylene or polypropylenewith from 0.01 to 10% by weight of an unsaturated carboxylic acid suchas maleic acid, acrylic acid or itaconic acid, or a derivative of anunsaturated carboxylic acid such as anhydrous carboxylic acid. If thecontent of the unsaturated carboxylic acid or its derivative is lessthan 0.01% by weight, the bonding property of the final adhesive tapewill be inferior, and if the content exceeds 10% by weight, the adhesionto a polypropylene-type sheet will be inadequate.

The acrylic adhesive is the one obtained by copolymerizing an acrylicacid ester as the major component with from 0.1 to 15 molar % of acompound having a functional group. As the compound having a functionalgroup, there may be mentioned a carboxylic acid, a carboxylic acidanhydride or a compound having a hydroxyl group such as acrylic acid,methacrylic acid, maleic anhydride or hydroxyethylacrylate. If thecompound having a functional gorup is less than 0.1 molar %, the bondingstrength between the adhesive layer and the bonding layer will beinferior, and if the amount exceeds 15 molar %, the adhesioncharacteristics tend to be inferior.

The molecular weight of the acrylic adhesive to be used in the presentinvention is preferably within a range of from 250,000 to 700,000 byweight average molecular weight (as measured by gel-permeationchromatography). If the molecular weight is less than 200,000, anadequate adhesion property is not obtainable, and if the molecularweight exceeds 700,000, the application to the base sheet becomesdifficult.

The cross-linking agent is a compound having at least two functionalgroups in its molecule which are reactive with the unsaturatedcarboxylic acid or its derivative in the modified polyolefin or with thefunctional groups in the acrylic adhesive, and as such a cross-linkingagent there may be mentioned an isocyanate compound, an aziridinecompound or an N-methylol compound. Further, a so-called blockedcross-linking agent may also be used in which the functional groups ofthe above mentioned cross-linking agent are chemically bonded with othercompounds and the functional groups will be freed at a high temperature.

In the step of mixing the acrylic adhesive and the cross-linking agentin the absence of a solvent, it is necessary to prevent gelation as faras possible, and by the use of said blocked cross-linking agent, it ispossible to readily reduce the gelation to such an extent that theoperation can be done without troubles. Further, it is necessary toconduct the mixing at a relatively low temperature and to apply themixture onto the base sheet as swiftly as possible.

According to the present invention, the adhesive material can be appliedinitially in a substantial thickness since the thickness of the adhesivematerial will be thinned in the subsequent stretching step. Accordingly,the above mentioned high viscosity adhesive material which can nototherwise be coated uniformly in a thickness of the usual adhesive layer(e.g. as thin as 20μ) of an adhesive tape, can be readily uniformlycoated by a usual fountain-type coater. After coating the adhesivematerial, the laminated sheet thereby obtained is biaxially stretched bya stretching apparatus at a stretching rate of from 2 to 10 times in thelongitudinal direction and from 4 to 10 times in the transversedirection. In a case where the adhesive material is applied to apreliminarily longitudinally stretched sheet, the coated sheet isstretched only in the transverse direction. By this stretching, thethick coating of the high molecular weight adhesive material can beuniformly thinned. Further, if desired, a release treating agent may beapplied to the back side of the polypropylene base sheet laminated withthe modified polyolefin. This coating is preferably carried out prior tothe stretching step.

According to this embodiment, the following superior effects areobtainable:

(1) By the specific modified polyolefin layer interposed between thepolypropylene base sheet and the acrylic adhesive material havingfunctional groups, it is possible to obtain an adhesive tape having agood bonding property and in which the modified polyolefin layer and theadhesive material are firmly bonded to each other. This phenomenon isbelieved to be attributable to the fact that both the modifiedpolyolefin and the adhesive material have functional groups.

(2) After coating the adhesive material on the base sheet, stretching iscarried out, and therefore, the adhesive material can be applied in asubstantial thickness initially. Accordingly, it is possible to use anacrylic adhesive material having a high molecular weight (namely a highviscosity) which used to be hardly coated without using a solvent,whereby it is possible to obtain an adhesive tape having a good quality.

(3) After coating the adhesive material on the base sheet, stretching iscarried out, and accordingly, the base sheet is relatively thick at thetime of the application of the adhesive material. During the stretchingoperation, a constant tension is imparted to the base sheet, whereby itis possible to prevent the shrinkage or the formation of wrinkles on thebase sheet, which used to be likely to occur during the application ofthe adhesive material or during the cross-linking treatment. Further, bythe heating at the time of stretching, not only the corss-linkingtreatment of the adhesive material can be done but also the bondingstrength between the modified polyolefin and the adhesive material canbe increased.

Referring to FIGS. 2 to 5, an advantageous process for the production ofthe adhesive tape of the present invention comprises simultaneouslyextruding the layers (A), (B), (C) and (D) to obtain an unstretchedlaminated sheet, and after coating an adhesive material thereon,biaxially stretching it at a temperature lower than the melting pointsof the A-layer PP and the B-layer PP and higher than the melting pointof the C-layer PP.

By this process, the layers (A) and (B) are stretched at a temperaturelower than their melting points, whereby the strength and severabilityare imparted thereto. At the same time, in the layer (C), no voids orcracks will be formed at the interface with the inorganic fillerparticles since the layer (C) is stretched at a temperature higher thanthe melting point of the C-layer PP, whereby a roughened surface layerhaving good transparency and superior writability can be formed.

As mentioned above, the streching is carried out at a temperature lowerthan the melting point of the polypropylene of the base sheet and higherthan the melting point of the propylene copolymer used for the surfacelayer.

By carrying out the stretching under such conditions, theparticle-containing surface layer does not substantially undergoorientation by stretching since the flowability of the propylenecopolymer increases at the time of stretching, whereby it is possible toavoid the formation of voids or cracks which are otherwise likely toform at the interface between the propylene copolymer and the fineinorganic filler particles due to the stretching stress. On the otherhand, the base sheet is oriented by the stretching, whereby the desiredstrength and tearability are imparted thereto. Further, it is necessaryto set the stretching rate so that the thickness of theparticle-containing surface layer after the stretching becomes to be notmore than the maximum particle diameter of the fine inorganic particlesused. This is intended to minimize the thickness of theparticle-containing surface layer to impart good writability withoutimpairing the photostatic property while securing the particles on theflat surface so as not to wholly embed them in the layer.

By controlling the stretching temperature and the thickness of theparticle-containing surface layer under such conditions, it is possibleto secure the fine inorganic particles to form a flat surface on theroughened surface layer of the final adhesive tape, and to obtain anadhesive tape having good writability and a white surface by virtue ofwhich letters written thereon can easily be read and being free of voidsor cracks, thus providing a superior photostatic property with a totalpercent light transmission of at least 90%.

Now, the invention will be described in further detail with reference toExamples.

EXAMPLE 1

A composition obtained by mixing an isotactic polypropylene having amelt index of 4.0 and a melting point of 167° C. and a methylpentenepolymer having a Vicat softening point of 160° C. in a weight ratio of75:25, was melt extruded to obtain an unstretched sheet. On one side ofthe unstretched sheet, a particle-containing surface layer obtained bymixing a propylene-ethylene copolymer having a melting point of 155° C.as the component (X), and calcium carbonate having an average particlesize of 3μ and a maximum particle size of 10μ as the component (Y) withor without a methylpentene polymer (hereinafter referred to as "PMP"), apolystyrene modified by a carboxylic acid anhydride (hereinafterreferred to as "modified PS") or a polymethyl methacrylate (hereinafterereferred to as "PMMA") as the component (Z) in the ratio as shown inTable 1, was laminated by extrusion, while on the other side of theunstretched sheet, a modified polypropylene containing 3% by weight ofitaconic acid was simultaneously laminated by extrusion, whereby athree-layer unstretched laminated sheet was obtained. Then, on themodified polypropylene layer side of the unstretched laminated sheet, anacrylic acid ester adhesive material was coated in a thickness of 800μwithout using any solvent. Thereafter, the coated laminated sheet wasstretched 6 times in the longitudinal direction and 6.7 times in thetransverse direction at a strentching temperature of 161° C., and thensubjected to heat treatment at a temperature of from 164° to 165° C. for5 seconds, whereby an adhesive tape having a total thickness of 60μ, wasobtained. The roughened surface layer had a thickness as shown inTable 1. The writability, finger-tearability, the photostatic propertyand the non-glossiness of each adhesive tape thus prepared are shown inTable 1.

The writability was determined by writing on the surface of the tapewith a pencil having a pencil hardness of 6H and evaluated on thefollowing basis: a readily writable tape was designated by ( ○O ), amore or less hardly writable tape which is considered to be at apractical lower limit level, was designated by ( ○ ), and a tape onwhich writing is almost impossible was designated by (Δ).

The photostatic property was determined by measuring the total percentlight transmittance of each adhesive tape in accordance with JIS K-6718.

The non-glossiness was determined by measuring the relative specularglossiness of each adhesive tape with an angle of incidence of 60° inaccordance with JIS Z-8741.

                                      TABLE 1                                     __________________________________________________________________________                                                    Photostatic                                                                   property                                                                            Non-glossiness          Roughened surface layer                         (Total per-                                                                         (Relative-                 Copolymers*                                                                            Calcium carbonate                                                                       Thermoplastic                                                                         Thickness   Finger-                                                                             cent trans-                                                                         specular                No.                                                                              (wt. %)  (wt. %)   resin (wt. %)                                                                         (μ)                                                                              Writability                                                                         tearability                                                                         mittance)                                                                           glossiness              __________________________________________________________________________    1  70       30        0       2     ⊚                                                                    ⊚                                                                    94    45                      2  85       15        0       4     ⊚                                                                    ⊚                                                                    95    45                      3  70       30        0       12    Δ                                                                             ⊚                                                                    85    45                      4  65       30        PMP 5   4     ⊚                                                                    ⊚                                                                    94    30                      5  50       30        PMP 20  4     ⊚                                                                    ⊚                                                                    94    10                      6  35       30        PMP 35  4     ⊚                                                                    ⊚                                                                    94     9                      7  50       30        PMMA 20 4     ⊚                                                                    ⊚                                                                    95    12                      8  50       30        Modified PS                                                                           4     ⊚                                                                    ⊚                                                                    95    11                                            20                                                      9  70       10        PMP 20  4     ○                                                                            ⊚                                                                    97    15                      10 30       50        PMP 20  4     ○                                                                            ⊚                                                                    90     7                      11 30       30        PMMA 40 4     ○                                                                            ⊚                                                                    85    15                      12 30       30        Modified PS                                                                           4     ○                                                                            ⊚                                                                    85    15                                            40                                                      13 30       30        PMP 40  4     ○                                                                            ⊚                                                                    85    15                      14 PP (m.p. 167° C.)                                                               30        0       4     ⊚                                                                    ⊚                                                                    85    45                         70                                                                         __________________________________________________________________________     *Propylene-ethylene copolymer                                            

From Table 1, it is seen that the adhesive tapes of Samples Nos. 1 and 2falling within the scope of the present invention are superior in thewritability, the finger-tearability and the photostatic property. It isalso seen that adhesive tapes of Samples Nos. 4 to 10 falling within thescope of the present invention are superior in the writability, thefinger-tearability, the photostatic properties and the non-glossiness.In Sample No. 3 wherein the thickness of the roughened surface layerexceeds 10μ i.e. the maximum particle size of calcium carbonate as thefine inorganic filler particles, the finger-tearability is satisfactory,but the writability and the photostatic property are inferior. InSamples Nos. 11 to 13 in which the amount of component (Z) is greaterthan the polypropylene copolymer, it has been found that voids andcracks are likely to form in the roughened surface and they are inferiorin the photostatic properties.

Further, from Sample No. 14 wherein a polypropylene having a meltingpoint of 167° C., which is higher than the stretching temperature, wasused for the roughened suface layer, it has been found that whiteningoccurred due to the formation of voids and cracks and the photostaticproperty was thereby degraded.

EXAMPLE 2

An unstretched laminated sheet of a thickness of 1600μ having astructure as shown in FIG. 2 was prepared with use of the followingmaterials.

Layer (A):

Isotactic polypropylene having a melting point of 167° C. (identified as"PP" in Table 2)

Methylpentene polymer having a Vicat softening point of 160° C.(identified as "PMP" in Table 2)

Layer (B):

The resins as identified in Table 2

Layer (C):

Propylene-ethylene random copolymer having an ethylene content of 3% byweight and a melting point of 151° C.: 55% by weight

Calcium carbonate having an average particle size of 3μ: 20% by weight

Methylpentene polymer having a Vicat softening point of 160° C.: 25% byweight

Layer (D):

Itaconic acid-grafted polypropylene copolymer having an acid content of3% by weight and a melting point of 151° C.

On the layer (D) side of the laminated sheet, an acrylic acid esteradhesive material was applied in a thickness of 800μ without using anysolvent, and then the coated laminated sheet was biaxially stretched 6times in the longitudinal direction and 7 times in the transversedirection at a stretching temperature of 161° C., and subjected tofurther heat treatment at 164° C. for 5 seconds.

In this manner, the adhesive tapes identified by Samples Nos. 4 to 10 inTable 2 were prepared.

The adhesive tapes thus obtained were composed of a layer (C) of 4μ, alayer (A) plus a layer (B) being 35μ, a layer (D) of 1μ, and an adhesivelayer of 20μ, and the total thickness was 60μ. The ratio of thethickness of the layer (B) to the total thickness of the base sheet(40μ) was as shown in Table 2.

In the same manner as above, adhesive tapes having a structure as shownin FIG. 3 were prepared (Samples Nos. 11 to 14). The same materials asabove were used for the layer (A) and the layer (C), and an itaconicacid-grafted polypropylene (acid content: 3% by weight, m.p. 165° C.)was used as the layer (B) which served also as the layer (D). (Namely,in FIG. 3, the same resin was used for the layers (B) and (D).)

The adhesive tapes thus obtained were composed of a layer (C) of 4μ, alayer (A) plus a layer (B) being 36μ, and an adhesive layer of 20μ, andthe ratio of the thickness of the layer (B) to the total thickness (40μ)of the base sheet was as shown in Table 2.

Adhesive tapes of Samples Nos. 1 to 3 where the layer (B) was omitted,were prepared in the same manner as above.

The dispenser severability (i.e. severability by a dispenser cutter),and the finger-tearability of the adhesive tapes thus obtained are shownin Table 2.

For the determination of the dispenser severability, each adhesive tapehaving a width of 18 mm was used, and with use of a commerciallyavailable dispenser having a cutter blade of a thickness of 0.35 mm andwith blade tips spaced with a pitch of 1.3 mm, the tape was cut bypulling it at an angle of 20° to the left or the right, and at an angleof 15° relative to the cutter blade.

The cutting was tried 50 times each in the right and left directions,and the tape whcih was satisfactorily cut without oblique tearing wasdesignated by ( ○O ), the tape which was satisfactorily cut with asuccess rate of at least 80% and thus practically useful, was designatedby ( ○ ) and the tape having a success rate of the proper cutting ofless than 80% was designated by (Δ).

The finger-tearability was determined by pinching each adhesive tapehaving a width of 18 mm, by finger tips of both hands and trying to tearit in the direction of its width without using the finger nails, andafter 20 times of the tearing trials, the finger-tearability wasevaluated in the same manner as in the case of the dispenserseverability. The tape which was hardly tearable was designated by (X).

                                      TABLE 2                                     __________________________________________________________________________                    Layer (B)                                                     Main Layer (A)                 Thickness                                      No.                                                                              PP (wt. %)                                                                          PMP (wt. %)                                                                          B-layer PP (m.p.)                                                                            ratio (%)                                                                           Dispenser severability                                                                   Finger-tearability            __________________________________________________________________________    1  100   0      None           0     ⊚                                                                         X                             2  95    5      "              0     ○   ○                      3  75    25     "              0     Δ    ⊚              4  "     "      Isotactic PP (167° C.)                                                                3     Δ    ⊚              5  "     "      "              5     ○   ⊚              6  "     "      "              12    ⊚                                                                         ⊚              7  "     "      "              30    ⊚                                                                         ○                      8  "     "      "              35    ⊚                                                                         Δ                       9  "     "      Itaconic acid-grafted                                                                        12    Δ    ⊚                              propylene copolymer (151° C.)                          10 "     "      Propylene-ethylene                                                                           12    ○   ⊚                              copolymer* (162° C.)                                   11 "     "      Itaconic acid-grafted                                                                        3     Δ    ⊚                              propylene copolymer (165° C.)                          12 "     "      Itaconic acid-grafted                                                                        12    ⊚                                                                         ⊚                              propylene copolymer (165° C.)                          13 "     "      Itaconic acid-grafted                                                                        30    ⊚                                                                         ○                                      propylene copolymer (165° C.)                          14 "     "      Itaconic acid-grafted                                                                        35    ⊚                                                                         Δ                                       propylene copolymer (165° C.)                          __________________________________________________________________________     *Ethylene content: 0.1% by weight                                        

Each of the tapes Nos. 1 to 14 was superior in its writability by thepencil, photostatic property and non-glossiness. Among them, theproducts Nos. 5 to 7, 10, 12 and 13 were most safisfactory in both thedispenser severability and the finger-tearability.

In Nos. 4 and 11 where the layer (B) was too thin and in No. 9 where themelting point of the B-layer PP was too low, the dispenser severabilitywas not adequately improved but the finger-tearability was superior, andin Nos. 8 and 14 where the layer (B) was too thick, the fingertearability was not so good, but the dispenser severability was good.

Further, it is seen that the tapes No. 1 to 3 where the layer (B) wasomitted, were good in either the dispenser severability or thefinger-tearability, but not in both.

EXAMPLE 3

A two-layer laminated sheet was prepared by an extrusion laminationmethod in which a sheet (hereinafter referred to as "PP-type sheet"having a thickness of 1,600μ and prepared by melt mixing 70% by weightof a polypropylene having a limiting viscosity of 1.85 as measured in atetraline solution at a temperature of 135° C. and an isotactic index of97% as represented by the component insoluble in n-heptane (boilingpoint) and a methylpentene polymer having a Vicat softening point of160° C., was laminated with one of the various modified polyolefinshaving a thickness of 50μ as shown in Table 3, as a bonding agent. Then,commercially available release treating agent was applied onto the backside of the PP-type sheet in a solid content thickness of 4μ by a rollcoater. Further, an adhesive material prepared by mixing at 120° C. anacrylic acid ester polymer (the major component of the adhesivematerial) having a weight average molecular weight of 440,000 andcontaining 6 molar % of carboxylic acid groups and, as a cross-linkingagent, an isophorone diisocyanate compound blocked withmethylethylketooxime (blocked cross-linking agent), was appliedimmediately after the mixing on the surface of the modified polyolefinlayer at a coating temperature of 120° C. in a thickness of 800μ. Thislaminated sheet was preheated and then stretched at a stretchingtemperature of 162° C. at a stretching rate of 6 times in thelongitudinal direction and 6 times in the transverse direction, andafter heat treatment at 165° C. for 5 seconds, it was wound up and agedat 40° C. for one week. Then, it was slit to obtain tapes having a widthof 18 mm.

The thickness of each layer of the tape thus obtained was as follows:the PP-type sheet layer of 40μ/the modified polyolefin layer of 1μ/theadhesive layer of 20μ. For the purpose of comparison, an adhesive tapewherein an ethylene-ethylacrylate copolymer (EEA) or astyrene-butadiene-styrene copolymer (SBS) was used instead of themodified polyolefin, and an adhesive tape which was obtained bysubjecting a biaxially stretched polypropylene base sheet to coronatreatment to imaprt a wet index of 40 dyn/cm (as measured by JIS K-6768)and applying the same adhesive material as in Example 3 in a thicknessof 20μ, were prepared.

The bonding properties (Note 1) of the adhesive tapes thus obtained werecompared as shown in Table 3.

Note 1:

Bonding property: With use of press rollers as stipulated in JIS Z-1522,the tapes were pressed with thier adhesive layers face-to-face and leftto stand at least one week. Thereafter, they were subjected to a T-typepeeling test in an atmosphere of a predetermined temperature (20° C. and0° C.) under the peeling speed of 300 mm/min, whereupon the rupturestates of the peeled surface were observed. The cohesive rupture of theadhesive layer was rated as good ( ○ ). The interfacial rupture betweenthe adhesive layer and the modified polyolefin layer or between themodified polyolefine layer and the PP-type sheet layer, and between theadhesive layer and the PP-type sheet layer, or the rupture of themodified polyolefin itself, was rated as no good (X). Further, a partialinterfacial rupture was rated as fair (Δ).

                  TABLE 3                                                         ______________________________________                                                                  Bonding                                                                 (wt.  properties                                          Kinds of the bonding agents                                                                          %)     20° C.                                                                         0° C.                            ______________________________________                                        Modified polypropylene, Maleic                                                                      0.005   Δ X                                       anhydride                                                                     Modified polypropylene, Maleic                                                                      0.01    ○                                                                              Δ                                 anhydride                                                                     Modified polypropylene, Itaconic acid                                                               2       ○                                                                              ○                                Modified polypropylene, Acrylic acid                                                                6       ○                                                                              ○                                Modified polyethylene, Acrylic acid                                                                 10      ○                                                                              Δ                                 Modified polyethylene, Acrylic acid                                                                 20      Δ X                                       Ethylene-ethylacrylate copolymer (EEA)                                                                      ○                                                                              X                                       Styrene-butadiene-styrene copolymer                                                                         ○                                                                              X                                       (SBS)                                                                         Corona treated product*       Δ X                                       ______________________________________                                         *When the adhesive material was coated, the biaxially stretched               polypropylene sheet underwent shrinkage and wrinkles were formed.        

From Table 3, it is seen that the tapes wherein modified polyolefinscontaining from 0.01 to 10% by weight of the unsaturated carboxylic acidor its derivative, have good bonding properties.

It is also seen that the tapes wherein the acrylic acid ester adhesivematerial was applied via the ethylene-ethylacrylate copolymer (EEA) orthe styrene-butadiene-styrene copolymer (SBS) have inferior bondingproperties at low temperatures.

Further, the corona treated product of the biaxially stretchedpolypropylene sheet has an inferior bonding property, and it is seenthat wrinkles are formed by the application of the adhesive material.

EXAMPLE 4

The same PP-type sheet of a thickness of 1,600μ as used in Example 3 wasused, and a modified polypropylene containing 2% by weight of itaconicacid and having a thickness of 50μ was laminated by extrusion to form atwo-layer laminated sheet. Then, on the back surface of the PP-typesheet, a release treating agent was coated in a thickness of 4μ, andthen an adhesive material prepared by mixing one of acrylic acid esterpolymers (the major component of the adhesive material) having variousweight average molecular weights as shown in Table 4 and containing 6molar % of carboxylic acid groups and the same blocked cross-linkingagent as used in Example 3° at 120° C., was applied immediately afterthe mixing on the surface of the modified polyolefin layer at a coatingtemperature of 120° C. in a thickness of 800μ. Thereafter, the laminatedsheet was subjected to the same stretching, heat treatment and agingtreatment as in Example 3, and then slit to form tapes. The tapesthereby obtained had superior bonding properties ( ○ at 20° C. and 0°C.). A high molecular weight acryl adhesive material which used to behardly applicable without using a solvent, can evenly be applied in auniform thickness without leading to shrinkage or formation of wrinkles.The holding forces (Note 2) and the peeling characteristics (Note 3) ofthe adhesive tapes are shown in Table 4.

Note 2:

Holding force: In accordance with JIS Z-1524, a sample having a tapewidth of 18 mm and a sticking fringe of 20 mm was stuck on a stainlesssteel and left to stand at room temperature for 24 hours. Thereafter, astatic load of 1 kg was put on it in a hot air drier at 40° C. and theholding force was represented by the time required for the sample tofall. In a case where the time for falling exceeded 2 hours, the holdingforce was represented by the length of the displacement upon expirationof 2 hours.

Note 3:

Cohesiveness: In accordance with JIS Z-1522, a tape of a width of 18 mmwas press-fixed on a stainless steel sheet and left to stand at roomtemperature for 24 hours. Thereafter, it was peeled in an atmosphere of20° C.×65% PH at a peeling speed of 300 mm/min at 180°, and the rupturestate of the peeled surface was observed. A case where no cohesiverupture i.e. no adhesive residue was observed on the stainless steelsheet, was rated as good ( ○O ), and a case where the adhesive remainedpartially was rated as fair ( ○ ).

                  TABLE 4                                                         ______________________________________                                             Weight average molecular                                                      weights of acrylic acid                                                  No.  ester polymers*  Holding forces                                                                            Cohesiveness                                ______________________________________                                        1    180,000          100 minutes ○                                    2    250,000          At most 0.1 mm                                                                            ⊚                            3    440,000          At most 0.1 mm                                                                            ⊚                            4    600,000          At most 0.1 mm                                                                            ⊚                            ______________________________________                                         *They were measured by the gel permeation chromatograph (GPC) method.    

As described in the foregoing, the present invention provides anadhesive tape having superior writability, finger-tearability, dispenserseverability, photographic property, non-glossiness and bonding propertyand a process for its production, and is extremely useful forapplication to a mending adhesive tape. The present invention is alsoapplicable to processes for the production of adhesive sheets or labels.

I claim:
 1. A process for producing an adhesive tape which comprises forming an adhesive layer on one side of a base sheet of a polypropylene resin and a surface layer of a polypropylene resin having a melting point lower than that of the polypropylene resin of the base sheet and containing fine inorganic filler particles, on the other side of the base sheet to obtain an unstretched laminated sheet, and stretching the laminated sheet at a temperature of at least the melting point of the polypropylene resin of the surface layer to bring the thickness of the surface layer to be not more than the maximum particle diameter of said fine inorganic filler particles.
 2. The process according to claim 1 wherein a bonding layer of a modified polyolefin containing from 0.01 to 10% by weight of an unsaturated carboxylic acid or its derivative is interposed between the adhesive layer and the base sheet and an adhesive material composed of a mixture of an acrylic adhesive having a functional group and a cross-linking agent is applied via the bonding layer onto the base sheet to form the adhesive layer without using any solvent.
 3. The process according to claim 2 wherein said acrylic adhesive has a weight average molecular weight of from 250,000 to 700,000.
 4. The process according to claim 1 wherein the stretching is carried out at a temperature lower than the melting point of the polypropylene resin of the base sheet and higher than the melting point of the polypropylene resin of the surface layer.
 5. The process according to claim 1, 2, 3 or 4 wherein the laminated sheet is biaxially stretched at a rate of from 2 to 10 times in the longitudinal direction and from 4 to 10 times in the transverse direction. 